|Length:||1,433.52 mi (2,307.03 km)|
|Existed:||1957 – present|
|South end:||I-8 / SR 15 in San Diego, CA|
| I-10 in Ontario, CA
I-40 in Barstow, CA
I-70 near Cove Fort, UT
I-80 in Salt Lake City, UT
I-84 in Riverdale, UT and Tremonton, UT
I-86 in Pocatello, ID
I-90 near Butte, MT
|North end:||Hwy 4 at Canadian border near Sweetgrass, MT|
Interstate 15 (I-15) is the tenth longest Interstate Highway, following Interstate 35, and it is the fourth longest north–south Interstate Highway in the United States. I-15 passes through the states of Montana, Idaho, Utah, Arizona, Nevada, and California, and from the Canadian border to San Diego County, close to the Mexican border. This Interstate Highway serves the major cities of Butte, Montana, Salt Lake City, and Las Vegas, Nevada. It also passes close to the urban areas of Helena, Montana, Ogden, Utah, Los Angeles County, California, and Orange County, California.
The stretches of Interstate 15 in Idaho, Utah, and Arizona have been designated as the "Veterans Memorial Highway".
Since the creation of Interstate 15, it has served as a long-haul route for North American commerce. It is now officially chartered for this purpose. From the junction of Interstate 515 in Las Vegas to the Canadian border, I-15 forms part of the CANAMEX Corridor, a High Priority Corridor, as a result of the North American Free Trade Agreement.
Local portions were built to connect the Inland Empire with San Diego in California, facilitate tourism access to Las Vegas, interconnect all of the Metropolitan Statistical Areas in Utah except for Logan, and provide freeway bypasses for Pocatello, Idaho Falls, and Great Falls.
Since the construction of Interstate 15, California, Nevada, and Utah have consistently ranked in the fastest growing areas of the United States. As a result, the route of I-15 has substantially increased in population and commuter traffic has increased the traffic burden on the freeway. Current population estimates are that more than 75 percent of the population of Utah, 19 percent of the population of California, and more than 70 percent of the population of Nevada live in counties where Interstate 15 is the primary Interstate Highway.
Similarly, in California, I-15 is seeing more commuter traffic due to the growth of the Mojave Desert communities of Hesperia and Barstow, California. In all of these states, I-15 has recently been or is currently in the process of being upgraded to increase capacity. The portions in Arizona, Idaho and Montana have retained their rural, long-haul character. Although Arizona has also grown substantially since the construction of I-15, this highway serves only the isolated corner of northwestern Arizona.
Due to this rapid area growth, the I-15 corridor is the focus of several mass transit projects. The Las Vegas Monorail, FrontRunner commuter rail system and TRAX light rail in Salt Lake City are mass transit lines loosely parallel to I-15 that are now in operation. Between Los Angeles and Las Vegas has long been proposed as a maglev train route; in 2004 the California-Nevada Interstate Maglev project held public meetings on the plan.
Route description 
This highway's southern terminus is in San Diego, California, at Interstate 8, although via California State Route 15, a southern extension of the freeway, the route connects to Interstate 5 just north of the Mexican border. The northern terminus is in Sweet Grass, Montana, at the international border between the United States and Canada, where it becomes Alberta Highway 4. It is 1,433 miles (2,306 km) long from San Diego to Sweet Grass.
North of its junction with the Riverside Freeway, State Route 91, in the Inland Empire near Corona, the route roughly follows the former routes of State Route 31. North of Devore, the highway follows the approximate alignment of historic U.S. Highway 66 along with U.S. Highway 91 and U.S. Highway 395. U.S. 395 breaks away at Hesperia and the route continues on a direct path to Barstow 35 miles (56 km) to the north. Meanwhile, the old alignments of U.S. 91 and U.S. 66 follow the Mojave River from Victorville to Barstow along the National Trails Highway. At that point, I-15 follows the old route of U.S. 91 exclusively as U.S. 66 turned east toward Needles. For many parts of the highway, high-voltage power lines, such as Path 46 and Path 27, almost all originating from the Hoover Dam, follow the freeway. Many of these link distant power stations to the Los Angeles metropolitan area.
The southern starting point of Interstate 15 was in 1947 planned to be in San Bernardino, at the interchange with the San Bernardino Freeway (then US 70/99), now I-10). This was logical as I-15 was following the old alignment of the historic Route 66 which passed through San Bernardino. The segment was completed accordingly.
But in 1964, legislation was later passed to extend the interstate to San Diego. But instead of extending the existing freeway from the I-10 interchange south, the California Department of Transportation drew a new segment in Devore that 'branched' off of the original alignment and bypassed San Bernardino altogether. This segment's alignment is generally northeast to southwest for about 15 miles (24 km). Then, in Fontana/Rancho Cucamonga, its directional alignment shifts to north–south where it eventually junctions with Interstate 10 (about 15 miles (24 km) west of the original interchange in San Bernardino). The segment that had been built from Devore to San Bernardino was retained as an interstate, but was re-numbered as Interstate 215. Note that during the construction of I-15's present alignment, and for some time afterwards, I-215 was numbered as I-15E, and its actual mileage would begin at Interstate 10. I-15 runs for a total of 287 miles (462 km) in California.
Interstate 15 begins in Primm, continues through Las Vegas along the Las Vegas Strip corridor. Then the interstate crosses the border with Arizona in Mesquite. The whole interstate in Nevada runs entirely in Clark County, for a distance of 123.8 miles (199 km).
I-15 just clips the northwestern corner of Arizona with a total length of 29.4 miles (47 km). The stretch is separated from the rest of the state and has one major exit, at Beaver Dam/Littlefield, Arizona. It includes a spectacular section where the road twists between the narrow walls of the Virgin River Gorge.
I-15 continues through Utah for 401 miles (645 km). It is the main north–south connection for the state. The highway approximately follows the old alignment of U.S. Highway 91 from St. George to Brigham City. The highway passes through the fast-growing Dixie region, which includes St. George, Cedar City, and eventually most of the major cities and suburbs along the Wasatch Front, including Provo, Orem, Sandy, Salt Lake City, Layton, and Ogden. Near Cove Fort, Interstate 70 begins its journey eastward across the country. The interstate merges with I-80 for about 3 miles (5 km) from South Salt Lake to just west of Downtown Salt Lake City and also merges with Interstate 84 from Ogden to Tremonton. Along nearly its entire length through the state, I-15 winds its way along the western edge of a nearly continuous range of mountains (the Wasatch Range in the northern half of the state). The only exceptions are when it passes through the mountains south of Cedar City and again north of Cove Fort.
I-15 passes through Idaho for 196 miles (315 km). Interstate 15 crosses the Utah state line in Oneida County. The highway runs through Pocatello, Blackfoot, and Idaho Falls, intersecting with Interstate 86. The last county in Idaho that Interstate 15 passes through is Clark County. Finally, the interstate reaches the Montana state line at Monida Pass.
Interstate 15 continues onward through 396 miles (637 km) of Montana through the cities of Butte, Helena and Great Falls, intersecting with Interstate 90, Interstate 115 and Interstate 315. At Sweet Grass, I-15 terminates upon crossing the Canadian border into the province of Alberta; however, I-15 signage is present on Alberta Highway 4 southbound from Lethbridge to the U.S.-Canada border.
Interstate 15 was constructed along the route of U.S. Route 91. Once Interstate 15 was relatively intact U.S. 91 was decommissioned, except for one part in Northern Utah/Southern Idaho where Interstate 15 instead followed the route of former U.S. Route 191.
Interstate 15 originally had two suffixed routes. In California, Interstate 15 had an eastern branch bypassing San Bernardino, which was designated Interstate 15E. I-15E was renumbered and is now Interstate 215. Present day routing of Interstate 15 in California was originally given "I-15W" as its title while it was under construction (the original asphalt portions from Temescal Canyon to Ontario Street were dubbed I-15W on maps until 1974 ), but was never officially signed as such . In Idaho, I-15 had a western branch near Pocatello that connected I-15 and I-84 (then I-80N). This highway was designated Interstate 15W. It is now the western I-86.
Redesignation of SR 15 in San Diego as I-15 might eventually occur when the freeway's interchange with SR 94 is upgraded to Interstate standards. The interchange currently has left-exits and blind merges, and is due to be updated with a long-awaited widening of both SR 15 and SR 94 in 2016. At that time SR 15 could most likely be decommissioned and re-signed as Interstate 15.
Major intersections 
Listed in order from south to north:
- Interstate 5 in San Diego
- State Route 94 in San Diego
- Interstate 805 in San Diego
- Interstate 8 in San Diego
- State Route 52 in San Diego
- State Route 163 in San Diego
- State Route 78 in Escondido
- Interstate 215 in Murrieta
- State Route 91 in Corona
- State Route 60 in Ontario
- Interstate 10 in Ontario
- State Route 210 in Fontana
- Interstate 215 in Devore
- Interstate 40, State Route 58 in Barstow
- Las Vegas Beltway ("I-215" to the east, "CC 215" to the west) in unincorporated Clark County (Paradise) near McCarran International Airport
- U.S. Route 95 (also U.S. Route 93 and Interstate 515 to the east) in Las Vegas, the "Spaghetti Bowl"
Interstate 15 does not intersect any other Interstate Highway in Arizona.
- Interstate 70 in Cove Fort
- Interstate 215 in Murray
- Interstate 80 in South Salt Lake; they merge for about 3 mi (5 km) into Salt Lake City
- Interstate 215 in North Salt Lake
- Interstate 84 in Ogden; they stay merged until Tremonton
- Interstate 90 in Butte; they are merged for 9 miles (14.5 km)
- Interstate 115 in Butte.
- Interstate 315 in Great Falls
Auxiliary routes 
- Inland Empire, California – I-215
- Henderson, Nevada – I-515
- Las Vegas, Nevada – I-215
- Salt Lake City, Utah – I-215
- Butte, Montana – I-115
- Great Falls, Montana – I-315
See also 
- "Table 1: Main Routes of the Dwight D. Eisenhower National System Of Interstate and Defense Highways as of October 31, 2002". USDOT Federal Highway Administration. 31 October 2002. Retrieved 20 July 2011.
- "Utah@Rocky Mountain Roads - Interstate 15". Aaroads. Retrieved 01-02-2008.
- "CANAMEX CORRIDOR, The safe, smart and secure corridor". Canamex Corridor Project. Retrieved 01.02.2008.
- "Population-Visitor Center- Utah.com". Utah Travel Industry. Retrieved 01-02-2008.
- "Utah Department of Transportation, Projects Under Construction". Utah Department of Transportation. Retrieved 01-02-2008.
- "District 1 Construction Report". Nevada Department of Transportation. Retrieved 01-02-2008.
- "Interstate 15 Major Improvements". California Department of Transportation.[dead link]
- Staff. "FRA to begin environmental study for California-to-Nevada Maglev project". Retrieved 2007-04-21.
- "State Route 15: Mid-City Bus Rapid Transit Project". Caltrans. December 2010. p. 1.
- Arizona Department of Transportation Project 015 MO 000 H577901C, sheet 73 of 103 - revised May 2005
- "entry for Interstate 215 California". Interstate-guide.com. Retrieved 2011-11-27.
- "entry for I-86 Western". Interstate-guide.com. Retrieved 2011-11-27.
|Wikimedia Commons has media related to: Interstate 15|
- [dead link]
- 2005 Rand McNally "The Road Atlas 2005" - newest feature- interstate mileage by state
- California Highways: SR 15
- Cal-NExUS: Route 15 North[dead link]
- Cal-NExUS: Route 15 South[dead link]